The following publications are representative of the most relevant prior art known to the Applicant at the time of filing the application.
______________________________________ U. S. Pat. Nos. ______________________________________ 1,159,264 November 2, 1915 Pfaff 2,465,672 March 29, 1949 Blaha 2,609,318 September 2, 1952 Swentzel 2,636,826 April 28, 1953 Nicholson 2,752,258 June 26, 1956 Swentzel 3,960,577 June 1, 1976 Prochazka 3,968,194 July 6, 1976 Prochazka 4,184,882 January 22, 1980 Lange 4,187,116 February 5, 1980 Lange 4,377,542 March 22, 1983 Mangels et al 4,388,085 June 14, 1983 Sarin et al 4,431,431 February 14, 1984 Sarin et al 4,467,043 August 21, 1984 Kriegesmann et al ______________________________________
The general process of bonding silicon carbide particles by reactive nitriding is described in U.S. Pat. No. 2,752,258 of June 26, 1956 to Swentzel. Relatively coarse silicon carbide granules are mixed with much finer silicon and optionally also silicon carbide powder, along with minor amounts of clays or other binding aids, then pressed to give a green body. The latter is converted to a finished refractory by exposure to heat and to nitrogen or a nitrogen-bearing gas such as ammonia under non-oxidizing conditions, normally for at least several hours. By reaction with the nitrogen content of the gas, the fine silicon and some of the fine silicon carbide if any is present are converted into silicon nitride. These basic materials and processes continue in commercial use today and are used in the present invention.
Because silicon nitride has a lower coefficient of thermal expansion than silicon carbide, as nitride bonded refractories cool from the process of manufacturing them, stresses or even porosity can develop as the silicon carbide shrinks more than the silicon nitride with which it is bonded. For the same reason, refractories of this type were widely observed in the early art to be sensitive to thermal shock. One method of ameliorating the thermal sensitivity is described in U.S. Pat. No. 2,609,318 of Sept. 2, 1952 to Swentzel: adding other metals, particularly iron, to the fine silicon powder used for reaction bonding. It was observed that such additions, specifically in the form of ferrosilicon or ferromanganese silicon, led to refractory articles with substantially less thermal sensitivity. A disadvantage was that the added metal oxides also sometimes led to formation of a glaze, capable of staining material in contact with it, on the surface of the refractory. Another teaching of the '318 Swentzel patent was avoidance of this staining problem by forming a refractory article with an exterior portion free from added glaze-forming metals around a core which did contain such metal.
Another expedient for reducing thermal sensitivity is taught in U.S. Pat. No. 2,636,826 of Apr. 28, 1953 to Nicholson, which is the one item of prior art now known to the applicant which is most closely related to the instant invention. Nicholson teaches the use of zirconia, zirconium or a zirconium compound, preferably from 3-7% by weight of zirconia, as part of the bond for nitride bonded silicon carbide refractories. Nicholson also taught that commercial grade silicon powder, which contained about two percent total by weight of oxides of other metals, notably iron, was preferable to purer silicon powder because nitriding was accomplished faster with the less pure silicon powder. Nicholson further taught that the preferable form of zirconium addition was zirconia stabilized by calcium oxide and the preferable method of mixing the zirconia with the other ingredients was by dry tumbling, followed by wet kneading after addition of bentonite gel, which "serves to take up the zirconium oxide and the finely divided silicon powder . . . and distribute them evenly and uniformly throughout the molding mixture." (Nicholson column 4 lines 48-53) Nicholson, although describing the products of his invention as suitable for a wide variety of uses, taught nothing explicit about the modulus of rupture or any other quantitative measure of mechanical strength of the products of his invention.
U.S. Pat. No. 2,465,672 of Mar. 29, 1949 to Blaha taught that zirconium silicate, formed by adding zirconia to silicon carbide powder covered with its natural surface coating of silica, could form a satisfactory bond for a polycrystalline silicon carbide refractory. No nitriding was involved in this teaching, however. A related earlier teaching was in U.S. Pat. No. 1,159,264 of Nov. 2, 1915 to Pfaff, who taught that approximately equal amounts of zirconia and silicon carbide could be mixed together and fired to produce a refractory material. Again, no nitriding was involved.
Another distinct but related type of refractory is exemplified by U.S. Pat. Nos. 3,960,577 of June 1, 1976 and 3,968,194 of July 6, 1976 to Prochazka. These patents describe refractories made by combining about 90% silicon carbide and about 10% silicon nitride powders, together with a boron compound as a densifying aid, and hot pressing the mixture to form a densified ceramic directly. No reactive nitriding is taught, and the primary object of the invention appears to be provision of refractories which have sufficient electrical conductivity to be amenable to electrochemical machining and electrical discharge machining. Still more remote prior art is represented by U.S. Pat. Nos. 4,184,882 of Jan. 22, 1980 and 4,187,116 of Feb. 5, 1980 to Lange; these teach hot pressed composites of silicon carbide, silicon nitride, and other materials, but the composites contain no more than 40% by volume of silicon carbide.
U.S. Pat. No. 4,467,043 of Aug. 21, 1984 to Kriegesmann et al represents the opposite extreme in composition; it teaches refractories composed of at least 98.8% by weight of silicon carbide bonded with an aluminum containing additive rather than with silicon nitride as in the instant invention.
A variety of densifying aids for silicon nitride taught in U.S. Pat. Nos. 4,377,542 of Mar. 22, 1983 to Mangels et al and 4,388,085 of June 14, 1983 and 4,431,431 of Feb. 14, 1984 to Sarin et al. In all of these patents, refractories with silicon nitride as the primary constituent are taught; silicon carbide plays no significant role if any. Zirconia is among the many densifying aids taught (by Sarin '431).
Relatively little attention is apparent in the prior art to the modulus of rupture (MOR) of refractories, even though this is a critical property with respect to the use of refractories as kiln batts or plates. Such batts or plates are relatively thin sheets used to support other materials while firing. It is economically advantageous for the batts or plates to be as thin as possible, so that as little as possible of the expensive space within the high temperature region of the kiln will be occupied by batts, freeing more of the kiln space for production use. Currently, length to thickness ratios of more than about 50 for batts or plates made by nitride bonding silicon carbide are impractical, because of the danger of breakage when such batts are loaded with typical kiln firing loads. This corresponds to a maximum MOR of about 7000 pounds per square inch (psi). An MOR of about 8000 psi at all temperatures between room temperature and at least 1450.degree. C. would significantly improve the loading factors available for kiln batts.
In principle, the increased MOR of batts could be achieved by higher pressure pressing of the green batt before firing, but in practice such pressing is difficult for large batts because of the great size and weight of the mold bands required, and the resulting higher density makes full nitridation very difficult.